1. Field of the Invention
The present invention relates to implantable bone prostheses; and, more particularly, to a bone-compliant femoral stem for canines or humans having compatibility with natural bone.
2. Description of the Prior Art
Many patents address issues related to canine femoral and/or human femoral stems. Some patents address adding geometrical features or cracks within the femoral stem to reduce the stiffness of the femoral stem and bring it closer to the stiffness of bone tissue.
U.S. Pat. No. 4,808,186 to Smith discloses a controlled stiffness femoral hip implant for use in the hip or other appropriate body joint. In a hip, a ball member fixed to the femur is rotatably engaged with a cup-shaped socket member fixed to the acetabulum of the pelvic bone. The ball member is mounted on one end of a femoral component, which has an elongated stem receivable in the intramedullary canal of the femur. The stem has a longitudinal channel, which lies in the coronal plane when the stem is in the implanted condition. The thickness of the stem channel is variable between the proximal and distal ends so as to affect the moment of inertia at any given location along its length to achieve stem flexibility, which substantially correlates to the flexibility of the bone. The medial side of the length of the implant is milled out to form a channel shaped stem cross section. The amount of material removed determines the resulting decrease in stiffness of the implant. The outside geometry remains substantially unchanged with the exception of the open channel on the medial side of the implant. Because of the reduction of the moment of inertia of the implant stem, it is more flexible. It also exhibits higher stem stresses upon loading of the implant. Therefore, a careful balance must be achieved between the amount of material removed from the stem and the expected stress levels expected by the particular size implant. However, this milling or material removal operation correspondingly decreases the load carrying capacity of the stem and its flexure behavior is subject to buckling and other non-uniform deformation mechanisms. At the contact location between bone and the implant, the modulus or stiffness has a very large change preventing proper load transfer to the bone. This, in turn, leads to crack formation at the bone-implant interface.
U.S. Pat. No. 4,990,161 to Kampner discloses an implant with resorbable stem, providing a biodegradable anchor selected from the group consisting of polyesters of glycolic acid, polyesters of lactic acid, polyamides of alpha-amino acids, unmodified polymers, modified polymers, calcium hydroxyapatite, tricalcium phosphate or polylactic acid for a permanent implant of a bone joint, e.g. hip, shoulder, knee or finger. The biodegradable anchor is an elongated member having an exterior surface, which tightly engages with a cavity in the bone and is substantially immovable within the cavity upon implant. The anchor is secured by means of a non-resorbable attachment to the permanent implant. This implant has a metallic nonresorbable permanent implant, which is covered by a resorbable anchor that is attached to permanent implant by non-resorbable attachment. The implant fits tightly into a bone cavity. As the resorbable anchor biodegrades, bone ingrowth occurs and the implant is permanently attached to the bone tissue. At this stage, the function of the permanent implant is over and the biodegraded anchor takes all the load and does not stress the bone tissue. The anchor is attached to the permanent implant by non-resorbable attachments. However, as the biodegradable anchor deteriorates and bone regrowth occurs, the anchor and its attachment is weakened. During this interim healing period, the implant is clearly unstable and is subject to movement causing bone-healing problems.
U.S. Pat. No. 5,314,492 to Hamilton et al. discloses a composite prosthesis. The composite stem for a prosthesis, particularly a hip prosthesis, includes a metal core and a fiber reinforced composite shell surrounding the core to provide modulus values that are similar to bone. The composite shell is composed of braided high tensile strength fibers and a bio-compatible thermoplastic resin having substantially equal strength in all directions i.e. isotropic. The shell comprises between 40 and 75 weight percent fiber and 25 to 60 weight percent resin. The stiffness of the prosthesis can be varied along the length of the stem. The metal core contributes to between 5 and 25 percent of the total flexural stiffness of the stem. The composite prosthesis is said to minimize the modulus mismatch of the stem portion of the component with the cortical shell of the femur. In use, the stiffness of the inner member (prosthesis) of the femur-prosthesis combination is said to be minimized; this forces the outer member (cortical shell) to carry a greater portion of the load and react closer to the normal physiological manner thereby reducing the problem of bone resorption. The composite prosthesis disclosed by the '492 patent comprises a metal core, fiber reinforced isotropic polymeric composite shell. The modulus of the composite shell is adjusted to be nearly equal to that of natural bone tissue. There is no bond between the polymeric composite shell and the metal core. Therefore, a separation may occur between the polymeric shell and the metal core, even when the composite shell attaches to bone tissue. Therefore, it is not clear how the metal provides any load bearing support.
U.S. Pat. No. 5,316,550 to Forte discloses a prosthesis having a flexible intramedullary stem. The metallic prosthesis implant for a hip or other joint has an intramedullary stem that has flexibility. Bending and torsional deformation under the influence of active load forces are comparable to that of the surrounding bone. The stem has a tapered bore that longitudinally extends from the proximal end to the distal end. In a second embodiment, the stem has a slit, the width of which also tapers from the proximal end to the distal end providing flexibility to the stem. This flexibility therefore distributes the loading forces from the joint more uniformly over the supporting cortical bone with the result that bone degeneration from stress shielding is minimized or eliminated. The stem of the flexible intramedullary stem has a taped bore providing progressively decreasing stem wall thickness from the proximal and distal end. In a second embodiment a slit is also provided which has a varying width in one side of the wall of the stem with a tapered bore. Both these features provide flexibility to the stem. The decreased wall thickness of the stem at the distal end may result in buckling or collapse of the stem during torsional or bending deformation. The presence of slit complicates the deformation behavior since the slit can open out under load. In any case, the modulus of the metallic stem member is essentially the same as the metal, and any bone contacting the metal surface is subject to shear separation. The configuration of the tapered bore and slits is said to improve the bending and torsional deformation of the stem when loaded.
U.S. Pat. No. 5,443,512 to Parr et al. discloses an orthopaedic implant device. The orthopaedic implant device is formed from a combination of different materials. It includes a body metal component and a porous metal surface layer for intimate contact with bone. Also included is a polymer in the form of a casing that includes adhesive characteristics for attachment to the body metal component and the porous metal layer. The body is a metal made from cobalt chrome and the porous metal surface layer is made from titanium. The preferred polymer casing is polyaryletherketone. This polymer manifests aggressive adhesive characteristics to the metallic surface of the cobalt chrome and the titanium following transformation to a heated state. The orthopaedic implant device contains a titanium metal core to which a porous layer is secured using PAEK polymeric adhesive. Such adhesive is said to produce a strong bond between the titanium core and the titanium porous layer. There is no disclosure relating to the elastic modulus of the porous layer infiltrated with a PAEK polymer. The elastic modulus of the porous layer is expected to be high and anisotropic due to the orientation of the titanium fibers in the porous body and its bond to a polymer. Therefore, its contact with bone tissue will result in shear separation at the interface.
U.S. Pat. No. 5,514,184 to Doi et al. discloses an artificial joint having a porous portion. The total hip replacement type artificial joint comprises a stem and a socket. The stem is provided with a head at the upper end and comprises a porous portion as an upper portion. A protector is located under the porous portion at the same height as the porous surface of the porous portion. An intermediate portion, which underlies the porous portion, has a trapezoidal cross section. A lower portion, which underlies the intermediate portion, has a circular cross section. The protector is provided along the peripheral edge of the outer cup and at the same height as a porous surface of the cup. Such arrangement is said to prevent drop-out of the particles from the porous surfaces at the time of insertion into bones of a living body and permit the joint to bear the patient's weight imposed after an operation. A porous portion protector is provided at the same surface level as the porous portion to capture any particles that may drop off from the porous portion when the artificial hip joint is inserted into a surgically prepared bone cavity. There is no means provided to provide a bone-compliant joint since all the materials used for the artificial hip joint are very high modulus metallic materials. Since the disclosure anticipates the drop out of the particles from the porous coating, the particles are not metallurgically bonded to the stem and the coating is thin, not a thick coating capable of grading the modulus. The assembly of particles provide no elastic modulus representing load carrying capability, since they fall apart easily.
U.S. Pat. No. 5,591,233 to Kelman et al. discloses metal/composite hybrid orthopedic implants. The hybrid implant comprises an intraosseous metal portion and an intraosseous composite portion. The composite portion is comprised of filaments nonlinearly disposed within a polymer matrix to produce a structure of variable modulus along its length. The composite portions may be secured to the metal portion by a taper lock, an adhesive joint, or a shrink fit joint. This metal/composite hybrid orthopedic implant comprises an intraosseous metal portion and an intraosseous composite portion. The composite portion is comprised of filaments nonlinearly disposed within a polymer matrix to produce a structure of variable modulus along its length. Neither the metallic portion nor the composite portion with fibers in a polymeric matrix have elastic modulus similar to that of natural bone. Besides, the fibers in the composite portion are longitudinally oriented and as result the elastic modulus varies as a function of direction.
U.S. Pat. No. 5,733,338 to Kampner discloses an implant having a reinforced resorbable stem. A prosthetic implant for a bone joint has an anchor formed of a resorbable sleeve reinforced with a nonresorbable core. Initially, the resorbable layer gives secure anchorage of the stem with an interference fit equivalent to that obtainable with the current state of the art in prosthetic joint replacement. Selected surface portions of the permanent implant component are porous to permit and direct bony attachment. After an optimal period of time elapses to permit sufficient bony attachment around the permanent, porous implant component, the polymer surrounding the metal core of the anchor slowly degrades and ultimately disappears. As a result, only the small diameter nonresorbable core remains in the medullary cavity of the bone. Although a small diameter core remains permanently within the medullary canal, no load bearing occurs through that core because it is spaced apart from cortical bone and, therefore, does not come in contact with the surrounding cortical bone structure. The core becomes essentially nonfunctional and the bone does not “realize” that any stem is actually present. In biomechanical terms only the permanent implant component forming the articulating surface of the joint remains as a permanent functional component and it is the only artificial component across which stresses are being transferred. Thus, the '338 disclosure provides an implant with an anchor including a nonresorbable core surrounded by a resorbable sleeve. The implant has a central non-absorbable metallic core surrounded by an anchor made from resorbable material. The anchor creates an interference fit between the implant and the bone cavity and slowly disintegrates as the bone ingrowth occurs. At this point, the central metallic core becomes detached and no longer carries any load allowing the bone ingrowth to carry all the loading. This method creates a dangerous situation when the implant is first incorporated since the metallic core is not in any manner attached to the anchor. Moreover, the ingrowth of the bone into the anchor progressively destroys the integrity of the resorbable layer, and the bone structure is yet to be built completely for load sharing. The presence of a non-load carrying metallic core reduces the overall cross section of the bone reducing its load capacity.
U.S. Pat. No. 5,935,172 to Ochoa et al. discloses a prosthesis having a variable fit and strain distribution. The joint prosthesis comprises a metallic body having a plurality of negative surface features such as through-slots, deep grooves, tunnels or pits, or valleys defined between projecting fingers or flutes. The metallic body constitutes the structural component of the prosthesis, such as a shell, plate or stem. A second bio-absorbable component filling at least some of the negative surface feature attaches to and extends the body to provide both a fit and a change in the initial stiffness. The bio-absorbable component protrudes from an external surface of the femoral stem by a distance of about 0.001 inches to 0.250 inches. The second part provides a time-evolving structural coupling, such that the prosthesis initially fits the patient's remnant bone to provide rigid fixation, while the mechanical properties shift with time in vivo to change its contact or loading characteristics. The femoral stem joint prosthesis may be modular and the first, or structural component, accommodates bio-absorbable second components of varying geometries and dimensions which fit a range of bore sizes, and achieve different stiffnesses or strengths affecting load or strain distribution. This prosthesis with variable fit and strain distribution is a two-part prosthesis. A first part of the prosthesis is made of bio-compatible metal, which is the metallic support member. A second sleeve inserted over the first member is made of biosorbable material. The tip of the prosthesis is produced as a series of separated spikes with the biosorbable material wedged therebetween. As the bone grows into this distal region, the reduced stiffness at the distal end is realized. Therefore, the prosthesis is initially tight fitting and becomes more stiffness friendly as the biosorbable material is absorbed by the growing bone. There is no reduction in modulus incompatibility of the bone with the prosthesis when it is implanted; rather a large time needs to be spent before the biosorbable distal end of the implant is absorbed.
U.S. Pat. No. 6,228,123 to Dezzani discloses a variable modulus prosthetic hip stem. The variable modulus prosthetic hip stem is insertable in an intramedullary cavity to support an articulation component, which includes a proximal neck portion and a distal root portion. The proximal neck is solid compatible metal such as titanium, cobalt chromium, stainless steel and extends for a length effective to reach into the cavity and couple to surrounding bone for load bearing engagement. The distal root portion includes a stranded cable of 10 mil cross section wires, which fills the bone cavity but flexes to avoid significant transfer of bending stresses. The cable is tightly bunched at its junction with the neck, providing a transitional degree of stiffness to its distal part, which is significantly more flexible and bends to accommodate natural displacement of the surrounding bone. The prosthesis has a section modulus characterized by three distinct regions. The proximal end region has the largest cross section and presents a stiff modulus of solid material, while the distal region is composed of strands and presents a flexible bending stiffness. An intermediate region of relatively short length where the cable attaches to the upper portion has a bending modulus that changes quickly from fairly stiff to flexible. The location of this portion may be varied by changing the relative lengths of the proximal solid and distal cable portions thus determining or limiting to a small local region the area of bone which may experience any stress shielding. The stiffness of the cable may also be varied by use of different gauge wires or the addition of welds or circumferential bands. In this variable modulus prosthesis hip stem the stiffness at the proximal end is high compared to natural bone and acts as a load-bearing member. The stiffness at the distal end is low compared to natural bone, allowing the natural bone to participate in bone support. The variable modulus prosthetic hip stem does not have similar modulus or stiffness to a natural bone at any section of the prosthetic hip stem, but merely supports loads locally at different locations along the length of the hip stem.
U.S. Pat. No. 6,312,473 to Oshida discloses an orthopedic implant system. This orthopedic implant system satisfies the biological, mechanical and morphological compatibility needs. A solid metal femoral stem and a solid metal acetabular head are at least partially covered with diffusion-bonded foamed-shaped sheet made of commercially pure titanium or titanium alloy(s). The open-cells in said foamed metal sheet are impregnated with biocompatible polymethyl methacrylate resin cement, which is reinforced with selected oxides including alumina, magnesia, zirconia, or a combination of these oxides along with an application of a small amount of a metal primer agent. This disclosure mainly covers an aluminum foam, which is available and projects properties of a titanium foam, which is said to be currently unavailable. The disclosure projects the strength, and the modulus of elasticity of the foam layer would be less than that of a bone and would require reinforcement by a biocompatible polymer such as PMMA. Since the foam is projected to be the weakest element, the orthopedic implant is not securely attached to the bone structure even when bone ingrowth occurs within the bone. Therefore, the orthopedic implant will experience movement under load, creating a dangerous situation.
U.S. Pat. No. 6,656,226 to Yoon discloses a plastic jacket for a cementless artificial joint stem and artificial joint having the jacket. The artificial joint pivotally connects a first bone to a second bone. A hole is formed in the second bone into which the artificial joint is inserted. The artificial joint comprises a head that connects with the first bone, a stem of a longitudinal length to fit into the hole of the second bone, a neck connecting the head to the stem and a jacket enclosing the stem. The jacket comprises an outer surface and a porous body. The body is inserted into the hole of the second bone so that substantially the entire outer surface of the jacket is in direct contact with inner walls of the hole. The jacket has a blind opening that receives most of the stem. The body of the jacket comprises a plastic material, and the outer surface of the body is rough, such that the jacket is adapted to adhere to the second bone by natural growth of the second bone onto the outer surface of the jacket without use of cement between the outer surface of the jacket and the inner walls of the second bone. This cementless artificial joint implant has a porous or rough plastic jacket, which slides over the metallic stem and is inserted into a hole in the second bone. The bone tissue is expected to grow into the rough porous external surface of the plastic jacket. Thus, the plastic with bone ingrowth may be retained by the bone tissue in time, but there is no attachment between the metallic stem and the plastic jacket, as a result, the overall cementless fixation only relies on the friction between the plastic jacket and the metallic stem for implant integrity.
U.S. Pat. No. 6,887,278 to Lewallen discloses a prosthetic implant having a segmented flexible stem exhibiting varying stiffness. The stem comprises an elongated core and segments extending outward from the core. The segments are spaced apart so as to define transverse grooves surrounding the core between adjacent segments. The longitudinal length of the grooves, and the microstructurally consistent implant materials used for the core and the segments are selected such that the stiffness of the stem varies from the proximal end to the distal end. Typically, the stiffness of the stem will be lower at the distal end such that the distal end of the stem bears less force when loaded and thereby transfers more load to the proximal end of the stem, which has a higher stiffness. As a result, stress shielding the load carried by the implant bypasses the proximal end of the stem and minimizes bone resorption adjacent the proximal end of the stem. It is therefore an advantage of the prosthetic implant is to provide a fixation stem with reduced or varying stiffness such that the stress shielding problems associated with the high mechanical stiffness of the stem can be minimized. In this segmented flexible stem the central core of the femur implant is provided with a series of tubular sections, which result in segments that are separated by a groove. The grooves reduce the stiffness of the femur implant. Optionally, the femur implant may be coated with a porous metallic layer to enhance bone bonding properties. In spite of the presence of segments and grooves, the overall stiffness of the femur implant is still very high compared to the stiffness of a bone tissue. In addition, the presence of grooves decreases the load carrying capacity of the femoral implant.
U.S. Pat. No. 6,913,623 to Zhu discloses a two-piece fused femoral hip stem. The two-piece femoral stem orthopedic implant includes a metal core having a first end, a second end, a first elastic modulus, and a first porosity. A proximal body is fused directly onto the metal core between the first and second ends. The proximal body has a second elastic modulus, which is less than the first elastic modulus, and a second porosity, which is greater than the first porosity. The porosity of the proximal body may vary throughout. The orthopedic implant minimizes the elastic modulus mismatch between the femoral stem and the surrounding bone material. To this end, a metal core has a first end, a second end, and a first elastic modulus. The metal core has a proximal body between the first end and the second end, the proximal body has a second elastic modulus which is less than the first elastic modulus. In this two-piece fused femoral stem the proximal porous body is said to have a lower elastic modulus and contacts the surrounding bone only in the proximal portion of the implant. In the distal end of the metallic core, no porous body is provided and therefore, there is direct contact between metal core and bone tissue. This significant mismatch in elastic modulus at the distal section takes away any benefit of reduced elastic modulus at the porous proximal section. This implantable prosthesis uses a metallic core with an inserted polymeric proximal body having elastic modulus similar to bone tissue. However, the bond between the polymeric proximal body and the metallic core may be inadequate to produce a successful orthopedic implant.
There remains a need in the art for femoral stems for canines or humans that are compliant with the stiffness and elastic modulus of bone tissue so that load is transferred between the bone and the implant without creation of bone separation or crack formation. When stiffness mismatch is present, the same load carried elongates the implant member differently than the bone tissue resulting in crack formation or implant separation. Needed in the art is a compliant elastic stiffness or modulus femoral stem for canines or humans that will provide load transfer without crack formation from the initial time period immediately after implant surgery to complete healing of implanted tissue by bone ingrowth. It would be particularly advantageous if, due to the intimate contact between the prepared bone and the bone compliant femoral stem during the healing period, a long term permanent bond between the implant and the underlying bone structure could be made possible, so that extended motion meeting the needs of a dog could be provided. It would also be advantageous if the implanted device were to function without corrosion or rejection reactions throughout the lifetime of the dog or human patient, since the implanted device is permanently placed within the bone structure.